1. Field of the Invention
The present invention relates to a laser beam amplifier for amplifying a seed laser beam, and a laser apparatus using the laser beam amplifier. Especially, the present invention relates to a driver laser apparatus for irradiating a target material with a laser beam to turn the target material into plasma in an extreme ultraviolet (EUV) light source apparatus.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 60 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there is an LPP (laser produced plasma) light source using plasma generated by irradiating a target with a laser beam (hereinafter, also referred to as “LPP type EUV light source apparatus”). The LPP type light source apparatus irradiates a target material, for example, tin (Sn) supplied into a vacuum chamber with a driver laser beam to excite the target material and generate plasma. From the generated plasma, various wavelength components including EUV light are radiated, and a desired EUV component among them is selectively collected by using a collector mirror (EUV collector mirror) and outputted to a device using EUV light such as an exposure unit.
The output higher than 100 W is requested for the EUV light source. For example, even in the case of an EUV light source for generating EUV light relatively efficiently by using a carbon dioxide (CO2) laser apparatus and a tin (Sn) target, higher efficiency of the CO2 laser apparatus for outputting a driver laser beam for plasma generation is required for use in industrial application.
As the driver laser beam for plasma generation, a pulse laser beam is used, and therefore, a master oscillator power amplifier type laser to be used as a driver laser apparatus includes, for example, a laser oscillator for generating a short-pulse CO2 laser beam and a laser beam amplifier for amplifying the short-pulse CO2 laser beam.
The laser beam amplifier has a discharging unit for exciting a CO2 laser gas containing carbon dioxide (CO2), nitrogen (N2), helium (He), and additionally, according to need, hydrogen (H2), carbon monoxide (CO), xenon (Xe), and so on by discharge. In the laser beam amplifier, a seed laser beam generated by the laser oscillator is amplified into a laser beam having desired energy. The amplified laser beam is focused by a laser beam focusing optics and applied to a target material such as tin (Sn), xenon (Xe), or the like.
U.S. Patent Publication US005386431A discloses a regenerative amplifier laser array for performing multipass amplification. FIG. 26 shows a laser beam amplifier disclosed in US005386431A. The laser beam amplifier as shown in FIG. 26 employs a multipass system in which an incident seed laser beam travels back and forth in a laser medium at plural times, and thereby, a large amplification factor can be achieved by efficiently utilizing the energy of the laser medium. Therefore, downsizing of a driver laser apparatus incorporating the laser beam amplifier can be realized.
In the multipass amplification type laser beam amplifier as shown in FIG. 26, it is necessary to make the optical path of the seed laser beam within the discharge region longer in order to improve the amplification efficiency. Accordingly, the optical system is constructed such that the seed laser beam is reflected by a rear mirror and a front mirror at multiple times to make the optical path of the laser beam within the discharge region longer.
However, since the optical path length becomes up to ten meters in the conventional multipass amplification type laser beam amplifier, if the optical axis of the seed laser beam in the incident position is slightly shifted relative to the reference optical axis as shown in FIG. 27, an error between the optical axes (a difference between the reference optical axis and the shifted optical axis) increases during reflection and amplification of the seed laser beam. As a result, the laser beam amplification is not efficiently performed due to the large shift of the optical axis of the laser beam to be amplified, and the output after amplification becomes lower. In the case where the optical axis is more largely shifted, the laser beam may be out of the exit window and not outputted from the laser beam amplifier as shown by dotted lines in FIG. 27.
Therefore, the configuration of the laser apparatus using the multipass amplification type laser beam amplifier has been extremely difficult for the following reasons: (1) it is difficult to properly provide the laser beam amplifier in the optical path of the seed laser beam; (2) it is difficult to correctly guide the seed laser beam to the provided laser beam amplifier; and (3) it is difficult to accurately make optical alignment of the entire laser apparatus. Further, even if the configuration of the laser apparatus is properly formed, the optical path within the laser beam amplifier is largely shifted from the reference optical path due to the slight shift of the incident position or incident angle. Accordingly, the energy utilization efficiency in the discharge region becomes lower, and the amplification state becomes unstable. As a result, the position accuracy of the focusing point and the focusing ability of the amplified laser beam become deteriorated. Furthermore, in the case where the multipass amplification type laser beam amplifier is used in the driver laser apparatus of the EUV light source apparatus, the following problems occur. That is, when the position of the focusing point of the driver laser beam is largely shifted, the target material is not irradiated with the laser beam and EUV light is not generated. Even if the target material is irradiated with the laser beam, the energy of EUV light and the stability of the energy become lower.